Apparatus and method for detecting a work piece in an automatic processing apparatus

ABSTRACT

A method and an apparatus for detecting the presence of a work piece in an automatic processing apparatus are described. Previously known detection devices cannot make a clear distinction between the presence and absence of the work piece under all conditions. An apparatus is therefore provided which contains an ultrasound transmitter, an ultrasound receiver and a detection device (controller). The detection as to whether a work piece is held in a holder in the processing apparatus is carried out by irradiating the holder with ultrasound waves, receiving the reflected ultrasound waves and detecting on the basis of the reflected ultrasound waves. The method can be used advantageously in particular in polishing machines for wafers, where the polishing cloth is distinguished considerably, in terms of its acoustic reflection capacity, from the wafer to be polished.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/DE00/00943, filed Mar. 24, 2000, which designatedthe United States.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] In modern production systems, automatic processing apparatus isincreasingly used, in which a work piece is processed without theintervention of a human operator. In pieces of apparatus which receivethe work piece to be processed from a unit connected upstream, it mustbe ensured that the transfer has been carried out, that is to say thework piece is also actually located in a holder provided for thepurpose. In pieces of processing apparatus that automatically pick awork piece from a pool of the same, the same task arises. Adding to thisis the fact that, to some extent even during the processing of a workpiece, the latter can be lost, for example by its slipping out of itsholder. This state must also be detected, in order that the apparatuscan interrupt the processing and react in a suitable manner, for exampleby informing the operator or halting further processing.

[0004] Various methods have been developed in order to establish thepresence of a work piece, such as optical detectors (cameras, lightbarriers) or electrical contacts. An optical method is described, forexample, in U.S. Pat. No. 5,823,853.

[0005] By using the example of a wafer polishing device, the intentionis to show in the following text how such detection mechanisms have beenimplemented in the prior art.

[0006] Wafer polishing devices are used for the polishing of wafers,monocrystalline, flat silicon disks which can be used for the productionof integrated circuits. The wafer is placed, in this case, on apolishing cloth and held by a polishing tool (polishing head,“carrier”). A polishing agent (“slurry”) is applied to the polishingcloth, and a relative movement is produced between the polishing clothand the wafer.

[0007] During the polishing process of the wafer, in rare cases, waferlosses on the so-called polishing platen may occur as a result ofvarious irregularities, such as tool problems, process instabilities,faults in the transport or in the provision of the wafers. In order tominimize the resulting damage to the wafer that has been lost and to thepolishing tool, and also to the substrate, or in the best case toprevent it entirely, rapid detection of a wafer loss is important inorder to be able to interrupt the polishing process.

[0008] Polishing machines of the prior art therefore to some extent haveoptical sensors which, using the different optical reflection capacityof the wafer, on the one hand, and the substrate (polishing cloth) onthe other hand, are able to detect a loss of the wafer. Depending on thesurface composition of the wafer or the color of the polishing clothused, however, these sensors are suitable to only a limited extent forthe reliable detection of the presence of a wafer, since they cannotalways distinguish between the wafer and the polishing cloth. They cantherefore not be employed universally and, from time to time, lead toerroneous detection. Furthermore, many of the polishing agents normallyused lead to a coloration of the polishing cloth, which results in achange in the reflection behavior. This effect occurs above all in thecase of polishing agents for metal polishing. As a result of theseproblems, the proportion of erroneous detections is increased, whichruns counter to the trouble-free and automatic processing of the wafersin the polishing machine.

[0009] International Patent Disclosure WO-A-98/42605 discloses a methodof detecting a work piece in which a surface of a piece of material isirradiated with ultrasound. In this case, by use of the reflected soundit is determined whether the material is located correctly or the wrongway round.

[0010] A further method of detecting the presence or absence of a waferhas been proposed in the prior art. In this method, the vacuum on thepolishing head is measured, in order in this way to detect a possibleloss of the wafer by a dip in vacuum that occurs.

[0011] Although it is possible, with this type of detection, to monitorthe wafer transport which is effected by the polishing head, detectionduring the actual polishing process is impossible in many cases, sinceit is often the case that, in order to achieve uniform removal, a gascushion is applied to the wafer from behind, between the wafer and thepolishing head, so that during the polishing operation, no measurablevacuum is available.

[0012] In summary, it is possible to say that at the present time thereis no method available for detecting the presence or absence of a waferin a polishing machine that satisfies all the requirements.

[0013] In addition to the wafer polishing systems described above, thereare also, in other processing devices for work pieces, problems withspecific, previously known detection methods, which restrict theiruniversal applicability.

SUMMARY OF THE INVENTION

[0014] It is accordingly an object of the invention to provide anapparatus and a method for detecting a work piece in an automaticprocessing apparatus which overcomes the above-mentioned disadvantagesof the prior art devices and methods of this general type, whichprovides a physically different type of approach to distinguishingbetween the work piece and holder or processing apparatus, and thereforepermits a distinction to be made in accordance with other physicalvariables.

[0015] With the foregoing and other objects in view there is provided,in accordance with the invention, a method of detecting a presence of awork piece in an automatic processing apparatus. The method includes thesteps of providing a holder for the work piece in the automaticprocessing apparatus; irradiating the holder and/or at least part of asurroundings of the holder with ultrasound waves; receiving reflectedultrasound waves; and using the reflected ultrasound waves to determineif the work piece is held in the holder or if the work piece is locatedoutside the holder.

[0016] In one aspect, the invention makes use of the different acousticreflection capacity of a work piece as compared with its surroundings,for example a gripping arm or a substrate.

[0017] In a further aspect, the invention is used in particular toprovide a wafer loss detection system that operates reliably.

[0018] In yet another aspect, the invention is intended to include theapplication of sound waves to the detection of the presence and absenceof a work piece to be processed.

[0019] In a further aspect, the invention provides work piece detectionmechanisms precisely for very flat work pieces.

[0020] The detection is preferably carried out by using the ratio ofemitted to received ultrasound waves. Processing of a work piece isusually not started if its absence is detected. However, in programmingterms, it may be expedient instead to determine the presence of the workpiece in the surroundings of the holder. However, these alternativescorrespond in terms of their result, since absence from the holderresults in a presence in the surroundings.

[0021] In an added mode of the invention, there is the step ofprocessing the work piece if its presence has been detected.

[0022] The irradiating step, the receiving step and the using step canbe carried out repetitively during the entire processing of the workpiece. The processing is preferably interrupted if the absence of a workpiece from the holder is detected.

[0023] The invention is in particular directed to applications in whichthe work piece is flat, for example a wafer. The holder has a supportplane for holding the wafer.

[0024] The invention provides that the holder has a polishing head and apolishing cloth within a polishing apparatus.

[0025] The invention is likewise directed to an apparatus for detectingthe presence of a work piece in an automatic processing apparatus,provision being made for an ultrasound transmitter for irradiating thearea of a work piece holder in the processing apparatus, at least oneultrasound receiver and a detection device for detecting the presence ofa work piece.

[0026] The ultrasound receiver preferably contains a means or isprogrammed for converting the intensity of the received ultrasound wavesinto an electrical signal. The detection device preferably containsmeans (or programmed) for determining the intensity of the ultrasoundwaves received by the ultrasound receiver, means (or programmed) forcomparing the intensity with the intensity of the ultrasound wavesemitted by the ultrasound transmitter, and means (or programmed) fordeciding about the presence or absence of a work piece.

[0027] The detection device can contain a control device for startingand interrupting the processing of a work piece. A plurality ofultrasound receivers can be present in one apparatus. Likewise, theapparatus can be equipped in such a way that the ultrasound transmittercan output directed ultrasound waves.

[0028] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0029] Although the invention is illustrated and described herein asembodied in an apparatus and a method for detecting a work piece in anautomatic processing apparatus, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

[0030] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1a is a block diagram of a detection apparatus in aprocessing apparatus having a gripping arm for holding a work pieceaccording to the invention;

[0032]FIG. 1b is a block diagram of the processing apparatus without thework piece;

[0033]FIG. 2a is a block diagram of a detection apparatus in a waferpolishing system with a wafer; and

[0034]FIG. 2b is a block diagram of the detection apparatus in the waferpolishing system after the loss of the wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The present invention makes use of an acoustic reflectioncapacity of work pieces, which always differs from the reflectioncapacity of the background, for example a polishing cloth in a waferpolishing system, or from the reflection capacity of an empty space,which exists in the absence of a work piece between the holders of agripping arm.

[0036] From an ultrasonic transmitter, ultrasound waves of a definedintensity are emitted and strike the work piece, if present, and/orstrike the surroundings, for example a polishing cloth, of the workpiece. From there, a portion of the ultrasound waves is reflected andreceived by an ultrasound receiver. A detection device, that is to say acontroller, for example a process computer, then carries out a detectionmethod in which a decision is made as to whether there is a work pieceat the proper point or not, use being made of the different reflectioncapacity of the work piece as compared with the holder or thebackground.

[0037] To this end, a ratio of the intensities of the emitted and thereceived ultrasound waves is preferably determined.

[0038] If no work piece can be detected, no processing step isinitiated. If the presence of the work piece could be detected, thecontroller can bring about or indicate the processing of the work piece.

[0039] If it cannot safely be assumed that no loss of the work pieceduring the treatment can occur, a repetitive measurement is preferablycarried out, which ensures continuous monitoring of the presence of thework piece.

[0040] Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1a thereof, there is shown in schematic form aconfiguration having a machine tool with a gripping arm 1, which holds awork piece 2. A spindle drive 3 with a milling head 4 is ready toprocess the work piece 2. An ultrasound transmitter 5 emits ultrasoundwaves (arrow), which are reflected by the work piece and pass to theultrasound receiver 6. They are converted by the receiver 6 intoelectrical signals, which are received by a controller 7 via a line 8and are evaluated there. The work piece 2 reflects a high proportion ofthe ultrasound waves that strike it, and therefore indicates itspresence.

[0041]FIG. 1b shows the same machine tool with identical referencesymbols, in which no work piece 2 is held by the gripping arm 1. In thiscase, only a small proportion of the ultrasound is reflected by thegripping arm 1, while the major part passes through its holder.Therefore, the ultrasound receiver 6 receives only a few ultrasoundwaves, so that the absence of the work piece 2 is detected.

[0042] The present invention can be used particularly beneficially inthe case of flat work pieces 2, such as the wafers described above. Inthe case of the wafers, the additional difficulty arises that, in thecase of flat work pieces, detection by light barriers or the like, forexample, is difficult.

[0043] The wafers are polished on polishing cloths that, in terms oftheir compressibility (elasticity), differ considerably from a waferlying on them. The compressibility of the polishing cloth iscomparatively high, which, on account of the associated sound absorptioncoefficient, leads to the sound waves being returned with a considerablylower intensity than in the case of the wafer alone. The polishing clothand the wafer can therefore be distinguished clearly and simply.

[0044]FIG. 2a shows schematically a polishing system for a wafer 12,having a polishing cloth 11 as the background and a wafer 12 disposedthereon. The detection device 7 has the same structure as in FIGS. 1aand 1 b, so that the same reference symbols are used. The wafer 12reflects a large proportion of the ultrasound radiation striking it, sothat the received intensity at the ultrasound receiver 6 is high.

[0045]FIG. 2b shows the polishing system without the presence of thewafer 12. The polishing cloth 11 reflects less ultrasound than the wafer12, so that correspondingly fewer ultrasound waves are received by theultrasound receiver 6. By using the intensity of the reflectedultrasound, given a known emission intensity, a wafer loss can thereforebe detected in a simple way, quickly, independently of the process (e.g.polishing paste, etc.) and independently of the hardware.

[0046] Commercially used wafer holders, carriers or polishing heads, asthey are known, often cover the wafer 12 completely from above.Ultrasound irradiation of the wafer 12 for the purpose of monitoring istherefore not possible. In such a case, it may be advantageous toirradiate the surroundings of the carrier, that is to say for examplethe polishing cloth 11. If the wafer 12 becomes detached from its holderin the carrier, it will slide out of the carrier as a result of thefriction between itself and the background, so that it then comes to lieon the polishing cloth 11 outside the carrier. In this case, thereflection capacity of the background at the point where the wafer islocated changes. This difference can be detected and, during theevaluation, can be judged to be a wafer loss, so that the processingapparatus can be stopped.

[0047] The controller 7 is used for the detection of the presence orabsence of the work piece at the envisaged position. It is equipped witha device to receive the electrical signals generated by the ultrasoundreceiver 6 via the line 8. In addition, it has a means that decideswhether a work piece is present or not. The controller can also has ameans that causes the ultrasound transmitter 5 to emit ultrasound. Thiscan be done via a further line 8′, which connects the controller 7 andultrasound transmitter 5 to each other. In this way, synchronization ofemission and measurement is possible, which reduces the influence ofpossible interference waves. Furthermore, the intensity of the emissioncan also be controlled and therefore defined clearly.

[0048] In a preferred embodiment, the controller 7 compares theintensity of the emitted ultrasound waves with the intensity of thewaves received by the ultrasound receiver 6, and can carry out thedetection of the work piece from their ratio. The various means used forthe control may be accommodated, for example, in a process computer.

[0049] The controller 7 can advantageously intervene directly in theprocessing of the work piece, by starting the latter and, if necessary,interrupting it. This can be achieved via conventional control linesbetween the controller 7 and the processing apparatus 3, 4.

[0050] In addition, it is possible to provide a plurality of ultrasoundreceivers 6 instead of one such receiver. This is advantageous in thecase of complexly shaped work pieces or if a plurality of work pieces,for example a plurality of wafers, are to be processed at the same time.

[0051] In addition, it is possible to operate with directed ultrasound,in order to permit still more accurate distinction between presence andabsence of a work piece. Directed ultrasound, which is emitted by aspecifically aimed ultrasound transmitter, permits more accurate echolocation of the location in the apparatus at which the work piece shouldbe found. In this way, the influence of other elements in the processingapparatus is minimized, which further improves the precision of thedecision process of the controller.

We claim:
 1. A method of detecting a presence of a work piece in anautomatic processing apparatus, which comprises the steps of: providinga holder for the work piece in the automatic processing apparatus;irradiating at least one of the holder and at least part of asurroundings of the holder with ultrasound waves; receiving reflectedultrasound waves; and using the reflected ultrasound waves to determineif the work piece is held in the holder or if the work piece is locatedoutside the holder.
 2. The method according to claim 1, which comprisesusing a ratio of emitted to received ultrasound waves for determiningthe presence of the work piece.
 3. The method according to claim 1,which comprises preventing a processing of the work piece if an absenceof the work piece is detected.
 4. The method according to claim 1, whichcomprises processing the work piece if the presence of the work piecehas been detected.
 5. The method according to claim 4, which comprisescarrying out the irradiating step, the receiving step and the using steprepetitively during the processing of the work piece.
 6. The methodaccording to claim 5, which comprises interrupting the processing if anabsence of the work piece from the holder is detected.
 7. The methodaccording to claim 1, which comprises processing a flat work piece asthe work piece.
 8. The method according to claim 7, which comprisesprocessing a wafer as the work piece.
 9. The method according to claim1, which comprises providing the holder with a support plane.
 10. Themethod according to claim 1, which comprises providing the holder with apolishing head and a polishing cloth in a polishing apparatus.
 11. Themethod according to claim 1, which comprises providing the holder with agripping arm.
 12. An apparatus for detecting a presence of a work piecein an automatic processing apparatus, comprising: an ultrasoundtransmitter for irradiating ultrasound waves over an area of a workpiece holder of the automatic processing apparatus; at least oneultrasound receiver for receiving reflected ultrasound waves; and adetection device for detecting a presence of the work piece andconnected to said ultrasound receiver.
 13. The apparatus according toclaim 12, wherein said ultrasound receiver converts an intensity of thereflected ultrasound waves received into an electrical signal.
 14. Theapparatus according to claim 12, wherein said detection device includesfirst means for determining an intensity of the reflected ultrasoundwaves received by said ultrasound receiver, second means for comparingthe intensity with an intensity of the ultrasound waves emitted by saidultrasound transmitter, and third means for deciding about a presenceand an absence of the work piece.
 15. The apparatus according to claim12, wherein said detection device includes a control device for startingand interrupting a processing of the work piece.
 16. The apparatusaccording to claim 12, wherein said ultrasonic receiver is one of aplurality of ultrasonic receivers.
 17. The apparatus according to claim1, wherein said ultrasonic transmitter can output directed ultrasoundwaves.
 18. The apparatus according to claim 12, wherein said detectiondevice is programmed to: determine an intensity of the reflectedultrasound waves received by said ultrasound receiver, compare theintensity of the reflected ultrasound waves with an intensity of theultrasound waves emitted by said ultrasound transmitter, and make adetermination about a presence or an absence of the work piece.